In-mold labeling has significant advantages over methods commonly used in the past to label plastic containers with polymeric labels. The most common of these previous methods involve the use of liner-carried pressure sensitive adhesive labels, or liner carried heat activatable adhesive labels. To produce the liner carried labels, a laminating step is performed to sandwich a layer of adhesive between a web of label stock and a web of silicone-coated paper which is to function as a carrier or release liner, the label stock is printed, the ink is dried by heating elements or ultraviolet radiation, separate labels are cut from the label stock by passing the combination through a rotary-die or flat-bed cutting station, and the matrix of waste or trim label stock surrounding the labels is stripped and discarded or recycled. Use of these types of methods results in high costs due to the use of a release liner, and the ecological difficulties in disposing of the liner and the trim.
In contrast, in-mold labeling avoids the use of any release liner or carrier. During in-mold labeling with polymeric labels, self-supported or free-film polymeric label stock is combined with heat-activatable adhesive, printed, die-cut and then arranged for deployment, as by being magazine-loaded as a series or stack of linerless labels, or by other means. The polymeric labels are then sequentially deployed on the molding surface of a blow mold to be bonded onto successive hot plastic substrates or containers. The blow molded parisons are expanded against the molding surface and the in-mold label which activates and bonds the heat-activatable adhesive to the blown plastic substrate or container.
When the in-mold label fails to form a bond with the plastic substrate, a blister may form. One of the main reasons for formation of the blister is that the heat seal layer did not activate completely or uniformly during the molding process. Another problem for in-mold labels is post-application blistering. This blister occurs when the heat seal layer bonds to the plastic substrate but fails to cool sufficiently fast enough to form a bond. The activation temperature of the heat seal layer affects the ability of the in-mold label to effectively bond with the plastic substrate.
Films for in-mold labels may be oriented or non-oriented. Films for in-mold labels may be stretched and oriented in a single or double direction. After stretching, the film is thermally set or annealed. If the film is not sufficiently annealed, some of the polymers of the label resume some of their original length. This reduction leads to shrinkage of the film label material. This shrinkage is a major contributor to gage bands and bagginess in the rolls of film material. Gage bands are formed from thickness irregularities in the film. As the film is stored on rolls, the thickness irregularities cause the roll to form what is referred to as gage bands. Bagginess is caused by relaxation of the polymers in the film upon storage. This relaxation leads to formation of “baggy” pockets or bagginess. The shrinkage of the material increases the effects of gage bands and bagginess. The defect makes the film unprintable and is thus a functional defect for in-mold labels.
It is desirable to obtain a label which solves one or more of the above problems. A label which is better bonding would have reduced blisters. Labels with reduced shrinkage and polymer relaxation would have reduced gage bands and bagginess. The usage of conventional polymers poses certain limitations on the manufacturability of in-mold label films. By nature, in-mold films have a layer of materials that activate under heat. However, these films need to be heated to stretch them and anneal them. The lowest melting fraction of the heat activated resin component determines the temperature of these process conditions, if the in-mold adhesive is applied prior to orientation as in coextruded film processes.
Conventional polymers have very broad molecular weight distributions, which means there will be fractions that have much lower melting points than the peak melting point. These fractions, called the low molecular weight tails, are significantly reduced in metallocene catalyzed polymers or in polymers which are distilled or fractionated to remove the low molecular weight tail. Hence polymers that have more ideal peak melting temperatures can be used, enhancing adhesive performance.
The low molecular weight tail mentioned above causes “plate-out” problems which require frequent down-times for cleaning up the machine. In “plate-out”, the low molecular weight tail separates from the main fraction of the film under the high temperature, pressure and shear in processing and condenses on the metal surface of rollers, dies and metal rolls. This condensed material attracts dirt, dust and suspended additives in the formulation and causes surface contamination. Thus, as the plate-out builds up on the machine, the machine needs to be stopped and cleaned periodically. Polymers without the low molecular weight tail, such as those prepared using metallocene catalysts, greatly decrease this downtime, from about once every 3 hours to about once every 24 hours, improving economics, throughput, productivity and reducing waste from shutting down the processing line. The reduced plate out allows an oriented film to be annealed at a higher temperature, which reduces film shrinkage.
Similar to the low molecular weight tail, there is also a high molecular weight tail present in polymers. This tail has a melting point which is substantially higher than the peak fraction. In the in-mold labeling process, as the hot plastic substrate or container comes into contact with the adhesive, the high molecular weight tail requires higher temperatures to activate. This higher temperature results in (1) very high cycle times since high temperatures also need long cooling times, which decreases productivity, or (2) non-uniform activation of adhesive, which causes blisters. In practice, both problems are encountered. Use of polymers with little high molecular weight tail, such as those produced with metallocene catalysts, alleviates this problem by ensuring uniform activation of adhesive.
In the application of in-mold labels, after the hot substrate contacts and activates the adhesive on the label, the whole assembly is rapidly cooled down to as close to room temperature as possible. During this rapid cooling process, the low molecular weight tail also needs to harden fast enough to provide a bond of substantial strength. If not, the differences in shrinkage rates of the container and label will result in post-application blisters. Polymers with lower amounts of low molecular weight tails, such as those produced with metallocene catalysts greatly decrease this phenomenon.
As an example, an EVA based adhesive with a 80° C. peak melting point has a broad molecular weight distribution which means that there will be low molecular weight tail that softens around, for example, 60° C., and high molecular weight tail that softens around, for example 100° C. This means the film needs to be annealed around 60° C. but the labels need to be heated to at least 100° C. for uniform activation. For comparison, a metallocene catalyzed polyolefin, with a peak melting point of 80° C. and a narrow molecular weight distribution, has a low molecular weight tail which might have a softening point of 70° C. and a high molecular weight tail that might have a softening point of 90° C. This film can be annealed around 70° C., which gives the film better properties, such as lower shrinkage and improved thermal stability. In the in-mold label process, the high molecular weight tail needs to be heated to only 90° C., ensuring better activation.